The present invention relates generally to lead assemblies for coupling implantable pulse generating medical devices with selected body tissue to be stimulated, and particularly, to tip electrodes forming part of such lead assemblies.
Although it will become evident to those skilled in the art that the present invention is applicable to a variety of implantable medical devices utilizing pulse generators to stimulate selected body tissue, the invention and its background will be described principally in the context of a specific example of such devices, namely, cardiac pacemakers for providing precisely controlled stimulation pulses to the heart. However, the appended claims are not intended to be limited to any specific example or embodiment described herein.
Pacemaker leads form the electrical connection between the cardiac pacemaker pulse generator and the heart tissue which is to be stimulated. As is well known, the leads connecting such pulse generators with the heart may be used for pacing, or for sensing electrical signals produced by the heart and representing cardiac activity, or for both pacing and sensing in which case a single lead serves as a bidirectional pulse transmission link between the pacemaker and the heart. An endocardial type lead, that is, a lead which is inserted into a vein and guided therethrough into a cavity of the heart, includes at its distal end a tip electrode designed to intimately contact the endocardium, the tissue lining the inside of the heart. An epicardial type lead includes a tip electrode in direct contact with the outside of the heart, that is, the epicardium. A myocardial lead has a tip electrode inserted into heart muscle, that is, the myocardium. In one typical form thereof, these leads include a proximal end having a connector pin adapted to be received by a mating socket in the pulse generator. A flexible, conductor, typically having a coiled configuration, is surrounded by an insulating tube or sheath and couples the connector pin at the proximal end of the lead and the tip electrode at the distal end of the lead.
The design of an implantable pacemaker pacing and sensing lead has a significant influence on the cost of its manufacture. For example, conventional pacing/sensing leads typically comprise several individual components which are assembled and welded together in a series of labor- and time-intensive, and therefore costly, steps in order to provide a reliable electrical and mechanical connection between the tip electrode and the associated electrical conductor.
With reference to FIG. 1, there is shown a simplified, axial cross section of an example of a conventional prior art lead assembly in the form of a unipolar, passive endocardial lead assembly 10 including a distal end portion 12. The lead assembly 10 includes an elongated lead body 14 extending along a central longitudinal axis 16 and covered by a tubular, flexible, biocompatible, biostable insulative sheath or housing 18 fabricated of silicone rubber, polyurethane, or other suitable polymer. As is known, the lead assembly""s proximal end portion (not shown in FIG. 1) is adapted to be electrically coupled to the receptacle of an implantable tissue stimulation pulse generator or pacemaker by means of a connector including an electrically conductive hollow pin. At the distal end portion 12 of the lead assembly, the insulative sheath 18 carries a plurality of outwardly extending tines 20 of known design for cooperating with the fibrous myocardial tissue to passively retain a tip electrode 22, comprising part of a tip electrode assembly 24, in position against the endocardium, in a fashion well known in the pacing art. The tip electrode 22 includes a proximal end 26 and a distal end 28, the latter having a distal extremity 29 defining an active tissue stimulating electrode surface 30 which, in the particular example shown, is generally hemispherical in shape. The tip electrode 22 includes a generally tubular section 31 having an interior chamber 32 which extends between the proximal and distal ends 26, 28 of the tip electrode. The distal end 28 of the tip electrode 22 may include one or more grooves 34 extending transversely of the longitudinal axis 16. A central bore 36 provides communication between the grooves 34 and the chamber 32 to form an elution path for the passage of a drug stored in a drug impregnated plug 38 enclosed within the chamber 32 of the tubular section of the tip electrode adjacent the distal end 28 thereof. The drug may be one intended to counter thrombus formation, fibrosis, inflammation or arrhythmias, or any combination thereof, or to accomplish any desired localized purpose. For example, the drug dispensing plug may be loaded with a steroidal anti-inflammatory such as dexamethasone serving to reduce the stimulation threshold by minimizing fibrotic encapsulation or fibrosis. The typical prior art tip electrode assembly 24 shown in FIG. 1 further includes a weld element 40 secured to the proximal end 26 of the tip electrode 22 by resistance or laser welding or the like. The weld element 40 has a first or distal projection 42 extending into the chamber 32 and a second projection 44, longer than the first projection 42, extending proximally. The second projection 44 includes a flange 46 and a tubular portion 48 extending proximally therefrom. The proximally extending tubular portion 48 of the weld element 40 extends into the lumen 50 of a coiled electrical conductor 52 which is held in place by means of a crimp tube 54, welds, or a combination thereof. As is known in the art, the coiled electrical conductor 52 extends through the lead body 14 to the proximal end of the lead assembly and is coupled to the aforementioned, hollow connector pin. Also, as is known in the art, the insertion and placement of the lead assembly 10 into the heart is aided by means of a stylet (not shown) passed through the hollow connector pin and lumen 50 of the conductor coil 52 and into the tubular portion 48 of the weld element 40 to enable the distal end portion 12 of the lead assembly 10 to be positioned at a desired location in the heart.
It will be seen that the prior art tip electrode assembly 24, which includes the weld element 40, comprises a lengthy, rigid structure making difficult the insertion of the lead transvenously into the heart and its maneuvering and placement therein. It will also be appreciated that there are substantial labor and material costs associated with the fabrication of the multiple component tip electrode assembly 24 of the prior art. For example, considerable time must be spent on resistance or laser welders in order to join the various components with the attendant costs of the skilled labor required.
Thus, there continues to be a need for pacemaker leads having tip electrode assemblies that comprise fewer parts, are simpler and less costly to manufacture and are smaller than existing tip electrode assemblies so as to facilitate insertion and placement of the tip electrode within the heart.
Tip electrodes in accordance with the present invention are extremely easy to manufacture and accordingly very cost effective in comparison to conventional tip electrodes. Indeed, the minimal expense of fabricating tip electrodes with superior electrical characteristics is a primary advantage of the present invention.
A feature of the present invention is the provision of a tip electrode which can be inexpensively manufactured while maintaining the performance achieved by much more expensive electrode designs.
In accordance with one, specific exemplary embodiment of the present invention, there is provided a body implantable lead adapted to transmit electrical signals between a proximal end portion of the lead and a distal end portion of the lead to thereby stimulate selected body tissue and/or sense electrical signals therefrom. The lead extends in a longitudinal direction and includes an elongated electrical conductor extending between the proximal and distal end portions of the lead assembly for transmitting the electrical signals. The conductor has an enlarged distal extremity or termination element. The lead further includes an electrically conductive tip electrode having a distal portion including a distal extremity defining an active exterior electrode surface. The tip electrode further has a proximal portion and a proximal extremity, at least the proximal portion of the tip electrode comprising a longitudinally extending, generally tubular structure having an interior bounded by a wall. The wall defines at least one through-aperture, the at least one through-aperture being smaller than the enlarged termination element at the distal extremity of the conductor. A longitudinal slot extends from the at least one aperture to the proximal extremity of the tip electrode, the enlarged termination element on the distal extremity of the conductor being disposed within the interior of the tube and being seated within the at least one aperture to provide a secure mechanical and electrical coupling between the conductor and the tip electrode. This coupling is preferably enhanced by laser welding the periphery of the at least one through-aperture to the enlarged termination element on the distal extremity of the conductor. Pursuant to one alternative embodiment of the present invention, the longitudinal slot extends distally from the at least one aperture.
In accordance with another embodiment of the present invention, the wall of the tubular portion of the tip electrode defines two through-apertures, the apertures being substantially diametrically opposed. In this embodiment, a longitudinal slot preferably extends from each of the apertures to the proximal extremity of the tip electrode. Further, each of the longitudinal slots may extend distally from the associated aperture. To enhance the mechanical and electrical coupling between the tip electrode and conductor, the enlarged termination element on distal extremity of the conductor may be welded to the tubular portion of the tip electrode along the periphery of at least one of the two through-apertures.
A drug dispensing element or monolithic controlled release device (MCRD) may be carried within the distal portion of the tip electrode for storing a drug to be dispensed to the body tissue. Such a drug dispensing member preferably takes the form of a plug adapted to be loaded with, for example, a steroidal anti-inflammatory such as dexamethasone which serves to reduce the stimulation threshold by minimizing fibrosis.
From the foregoing, it will be appreciated that the present invention provides an implantable lead having a tip electrode and tip electrode/conductor assembly that are easy and cost effective to manufacture. Moreover, the tip electrode of the invention, because it effectively eliminates the weld element and related structure of the prior art, has a reduced length thereby facilitating its insertion, maneuverability and placement within the heart.